Find longest subsequence in binary tree
Last updated: May 12, 2026
Quick Overview
Given a binary tree, write a function to find the longest subsequence of nodes where each node's value is greater than the value of its parent node. The function should return the length of this longest subsequence. The input will be the root node of the binary tree, and the output should be an integer representing the length of the longest increasing subsequence.
Postmates
May 12, 202611
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Given a binary tree, write a function to find the longest subsequence of nodes where each node's value is greater than the value of its parent node. The function should return the length of this longest subsequence. The input will be the root node of the binary tree, and the output should be an integer representing the length of the longest increasing subsequence.
Postmates uses this problem in the Phone Screen to evaluate your algorithmic thinking. They expect you to discuss multiple approaches, analyze trade-offs between them, and implement the optimal solution with clean, readable code.
What the Interviewer Expects
- Quickly identify the optimal approach and its theoretical basis
- Handle complex algorithm design with multiple interacting components
- Write concise, elegant code under time pressure
- Prove correctness of your approach and discuss alternative solutions
- Optimize beyond the obvious: discuss constant factor improvements
- Address follow-up variations and explain how the solution generalizes
Key Topics to Cover
How to Approach This
- Clarify input constraints and edge cases before writing code.
- Walk through your approach verbally and confirm with the interviewer before coding.
- Start with a brute force solution, then optimize. Mention time and space complexity.
- Test your solution with examples, including edge cases like empty input or duplicates.
- Consider common patterns: sliding window, two pointers, hash map, BFS/DFS, dynamic programming.
Possible Follow-up Questions
- How would your solution change if the input was sorted?
- Can you solve this iteratively instead of recursively (or vice versa)?
- What is the worst-case input for your solution?
- How would you modify your solution to handle streaming input?
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Practice DSA ProblemsSample Answer
Problem Analysis
To solve the problem of finding the longest increasing subsequence in a binary tree, we can utilize a Depth-First Search (DFS) approach. The primary pattern here is tree traversal, where we explore ea...
Approach
- Initiate DFS: Start the DFS from the root of the binary tree.
- Recursive Function: Create a helper function that takes the current node and the value of its parent as parameters.
- **B...
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